The present invention relates to a synthetic resin pipe having excellent pressure-withstanding properties against inner/outer pressures, which is used mainly as a city water pipe, a sewage pipe, various kinds of fluid transportation pipes, an underdrain pipe, a cable protective pipe and so forth, each of which is embedded in the underground. More particularly, the present invention relates to a synthetic resin pipe preferably employable as a city water pipe and a high pressure fluid transportation pipe.
A synthetic resin pipe in which a pipe wall has a metallic belt plate exhibiting a U-shaped cross-section embedded therein so as to improve properties withstanding flattening pressures has been already proposed by the present applicant and applications for such pressure-withstanding synthetic resin pipes were filed with the Japanese Patent Office by the applicant. At present, some applications have been already laid open (for example, see Japanese Utility Model Unexamined Publication No. Sho 61-141889).
Such publicly known synthetic resin pipe in which a pipe wall has a metallic belt plate exhibiting a U-shaped cross-section embedded therein, has been commercialized as synthetic resin pipe having excellent properties withstanding flattening pressures, and has been taken seriously and been widely used in various utilization fields.
However, even with this synthetic resin pipe in which a wall has a metallic reinforcement belt plate exhibiting a U-shaped section embedded therein, from the results derived from a series of tests conducted for checking a flattening strength of the pressure-withstanding synthetic resin pipe under the influence of a high load of 3000 kg/m exerted on the upper surface thereof, it has been found that the pipe has a problem that the strength of the synthetic resin pipe is not always satisfactory. For example, a series of tests for checking the flattening strength of a synthetic resin pipe P constructed as shown in FIG. 11 under the condition of an outer load of 3000 kg/m exerted thereon (wherein the synthetic resin pipe P was such that a raw material of synthetic resin employed therefor was polyethylene, an outer diameter of the synthetic resin pipe was 880 mm, an inner diameter of the pipe was 800 mm, an inner pipe wall 103 located inside a top portion 102 had a thickness of 2 mm, a synthetic resin at the top portion 102 had a thickness of 3 mm, an iron plate 1 had a thickness of 1 mm, a distance between open ends of the iron plate 101 was 61 mm, and a sample P of synthetic resin pipe to be tested had a length of 1 m) were conducted, and it was found as the results derived from the tests that a quantity of deformation of the synthetic resin pipe P (i.e., a quantity of sinking) from the no-load state (i.e., the state that the synthetic resin pipe P exhibited a true circle) was 44 mm, the distance between side portions of the pipe at the open ends of the iron plate 101 was enlarged, the inner pipe wall 103 located inside the top portion 102 was elongated in the axial direction of the pipe, and cracks occurred on parts of the inner pipe wall 103. However, any abnormality was not recognized with a bottom portion 104 between the adjacent top portions 102.